1. Field of the Invention
The present invention relates to a sliding part having a sliding surface of which wear resistance is required, such as a shim used in a valve system of an internal combustion engine of a vehicle, and to a sliding mechanism including the sliding part.
2. Description of the Background Art
A material having high wear resistance has been used for a mechanical sliding part represented by an engine part of a vehicle, in order to minimize wear. Recently, a hard sliding member comes to be formed of a material such as a super hard alloy, or ceramics having superior wear resistance as compared with the steel material which has been conventionally used as a material for the sliding part. These materials, however, are generally difficult to process and are expensive. Therefore, it is a common practice to form not the entire part but only a sliding portion requiring high wear resistance by using such a material.
As a representative example, on an end surface of a valve lifter driving a tappet valve of a valve system in the internal combustion engine, a shim formed of a hard member is positioned, which shim exhibits superior wear resistance.
As an example of a sliding mechanism for heavier load, a hard member is used at a tip end of a thrust bolt used for preventing inclination of a ring gear in a reduction mechanism of a vehicle. For example, Japanese Patent Laying-Open No. 8-109956 discloses means having superior durability and allows easy maintenance, for preventing inclination of the ring gear used in a reduction mechanism of a large vehicle such as a bus, a truck, a tractor or the like.
Referring to FIG. 10a, a reduction mechanism 100 contains, in a differential carrier 101, a propeller shaft 102 including a pinion, and an axle shaft 103 including a differential gear with a bearing (not shown) interposed. In a differential case 104, a ring gear 105 is secured. Ring gear 105 transmits torque of propeller shaft 102 to axle shaft 103. In order to prevent deflection of ring gear 105 when the transmitted torque increases, a tip end of a thrust bolt 106 is in contact with a rear surface 107 of ring gear 105.
FIG. 10B is an enlarged view of a portion around thrust bolt 106 and rear surface 107 of ring gear 105. A boss 108 is provided at a part of differential carrier 101, thrust bolt 106 is screwed in boss 108, and thrust bolt 106 is positioned by using a lock nut 109. At a tip end of thrust bolt 106, a sliding part 110 is mounted.
FIG. 10C shows, in further enlargement, the periphery of sliding part 110. Sliding part 110 having a sliding surface crowned to have a convex shape and formed of silicon nitride or the like is mounted on a recessed portion 106a of thrust bolt 106. Between the sliding surface of sliding part 110 and rear surface 107 of ring gear 105, there is generally a clearance of .delta.. When an excessive torque is transmitted to reduction mechanism 100 and ring gear 105 deflects by more than .delta. when the vehicle starts or climbs a steep slope, thrust bolt 106 prevents inclination of more than .delta.. Therefore, during normal running, sliding part 110 does not contact rear surface 107 of ring gear 105.
By the structure of the reduction mechanism, inclination of ring gear 105 by more than .delta. can be prevented, and therefore abnormal wear of ring gear 105 and the teeth surface of the pinion of propeller shaft 102 or damage to the teeth can be avoided. Further, as the sliding surface of sliding part 110 is crowned to have a convex shape, sliding part 110 is in smooth contact with the rear surface 107 of ring gear 105, and therefore it is described that a force that would cause damage or displacement of sliding part 110 from the recessed portion 106a is hardly generated.